1. Field of the Invention
The invention relates to a device for reversibly gripping a bolt-shaped element, in particular a rivet mandrel of a blind rivet. The invention furthermore relates to a method for reversibly gripping such a bolt-shaped element. The device has a tension head which extends in an axial direction and has a mouthpiece sleeve, a clamping jaw sleeve which is movable in the axial direction in the mouthpiece sleeve, and a plurality of segmented clamping jaws each extending in the axial direction from a front end to a rear end. The clamping jaws form between one another a receiving space for receiving the bolt-shaped element, are arranged within the clamping jaw sleeve, and are formed such that they are displaced in a radial direction in order to reversibly grip the bolt-shaped element in the event of an axial movement of the clamping jaw sleeve.
A device of the generic type is described in German patent No. DE 100 29 392 B4.
When a blind rivet is set into a component, the blind rivet is introduced with a blind rivet sleeve into the pre-punched component until it comes to rest against the component by way of a setting head. A rivet mandrel is guided through the rivet sleeve, the rivet mandrel bearing on one side against the end sleeve by way of a mandrel head. During the setting operation, the rivet mandrel is gripped with the aid of a setting device and is pulled in an axial direction such that a closing head is formed opposite the setting head and the blind rivet is fixedly connected to the component.
During this setting operation, reliable gripping and pulling of the rivet mandrel in the axial direction is necessary. When a desired setting force is reached, the rivet mandrel breaks off.
For a high-quality blind rivet connection, exact and defined gripping of the rivet mandrel is necessary. This applies in particular to automated blind rivet setting methods, for example in production lines in which the blind rivets are set in a fully automated manner with the aid of blind rivet robots. Such blind rivet robots are increasingly used nowadays, for example in the motor vehicle industry during car body manufacturing.
For gripping and pulling, generally—also according to the German patent DE 100 29 392 B4—so-called tension heads are known which extend in the axial direction and have a mouthpiece sleeve by way of which the tension head is supported on the component surface during the setting operation. Arranged within the mouthpiece sleeve is a clamping jaw sleeve which is arranged so as to be movable in the axial direction in the mouthpiece sleeve. The clamping jaw sleeve narrows in this case at its front end side in a wedge-like or conical manner and receives, in this front part region, clamping jaws which are formed in a correspondingly conical or wedge-like manner and are designed to receive the rivet mandrel. Via a drive of the setting appliance, for example a hydraulic drive or an electric drive, the clamping jaw sleeve is displaced toward the rear in the axial direction during the setting operation, such that the clamping jaws, on account of the conical configuration, are displaced radially inward and grip the rivet mandrel, previously introduced through a mouthpiece, in a clamping manner and subsequently pull the rivet mandrel toward the rear in the axial direction until the required setting force has been reached and the rivet mandrel breaks off.
In the (fully) automated setting process, it is necessary for the respective residual mandrel to be received automatically in the tension head, for automatic movement to the desired setting position to take place and subsequently also for the broken-off residual mandrel to be automatically removed in a technically reliable manner. During gripping, it is possible in particular for problems to occur in this case when the rivet mandrel is not oriented exactly axially parallel with a center axis of the tension head. The rivet mandrel is in that case not accommodated concentrically within the clamping jaws, and as a result, during the subsequent pulling-back process, the clamping jaws clamp the rivet mandrel only at different axial positions and this can result in an axial offset of the clamping jaws. This generally results in a nonuniform distribution of tensile force in the clamping jaws, such that one part of the clamping jaws is subjected to greater loading than the other part, resulting in premature wear.
A further problem is the reliable disposal of the broken-off residual mandrel. In this case, there is sometimes the risk that the residual mandrel remains adhering to the clamping jaws after the setting operation has been completed and after the clamping jaws have been moved back into their front starting position. This is a considerable disadvantage in particular in fully automated methods, since the production process then has to be interrupted.
In such automated operations, furthermore, the automated gripping of the blind rivet also represents a certain problem since the rivet mandrel of the blind rivet has to be introduced into the clamping jaws with the latter open. In order to prevent the rivet mandrel from slipping and falling out, the latter has to be held in the mouthpiece sleeve via a special measure until the clamping jaws exert a certain clamping force on the rivet mandrel in order to secure the latter against falling out before the setting appliance moves the blind rivet to the designated position and inserts it into the pre-punched component.